1. Field of the Invention
The present invention relates generally to magnetoresistive elements, and more particularly to a magnetoresistive element having a so-called CPP (Current Perpendicular to Plane) structure that causes a sense current to flow in the direction of film thickness using a spin-valve (SV) film, and a method of manufacturing the same.
2. Description of the Related Art
In recent years, giant magnetoresistance (GMR) elements having a multilayer structure film of two ferromagnetic layers and a nonmagnetic metal film of, for instance, Cu sandwiched therebetween have been developed. GMR uses spin-dependent scattering in the ferromagnetic layers and at the interfaces. That is, GMR uses a property such that when the directions of magnetization of two magnetic layers are parallel, conduction electrons having spin in a certain direction are less likely to be scattered, thus resulting in low resistance; while the conduction electrons are more likely to be scattered, thus resulting in high resistance, when the directions of magnetization are antiparallel.
In magnetoresistive elements using a multilayer SV film, an antiferromagnetic body is brought close to one of two ferromagnetic layers so as to fix its direction of spin (a pinned layer), while the direction of magnetization of the other ferromagnetic layer is caused to be easily changeable with respect to an external magnetic field (a free layer). Using a property where the element resistance changes depending on the relative angle of the direction of magnetization between the two magnetic layers, the direction and size of an external magnetic field can be detected based on a change in the element resistance.
Such magnetoresistive elements are applied to and used in practice in magnetic sensors and the reproduction heads of hard disk drives.
In the conventional magnetoresistive element using an SV film, a resistance change in the in-plane direction of the SV film is detected by causing a sense current to flow in the film in-plane direction. Such a structure is referred to as CIP (Current In Plane) structure.
On the other hand, a magnetoresistive element of the CPP structure causing a sense current to flow in the direction of film thickness of an SV film to detect a resistance change in the film thickness direction has drawn attention as a′ magnetoresistive element having higher density and higher sensitivity. The CPP magnetoresistive element, which has a characteristic such that the element output increases as the element size decreases, is promising as a highly sensitive reproduction head in high-density magnetic recorders (for instance, Atsushi Tanaka et al., “Spin-Valve Heads in the Current-Perpendicular-to-Plane Mode for Ultrahigh-Density Recording,” IEEE Trans. Magn., Vol. 38, pp. 84-88, Jan. 2002).
Further, a CPP magnetoresistive element of a tunnel magnetoresistance (TMR) type that applies a perpendicular current to a tunnel junction film similar in structure to the SV film is also known (for instance, Japanese Laid-Open Patent Application No. 2003-198005).
In a CPP reproduction magnetic head using a magnetoresistive film such as an SV film or a TMR film, formation of the magnetoresistive film and formation of an upper shield or an upper terminal are not successively performed in the process of its shape formation.
FIG. 1 is a diagram showing a conventional CPP-SV (TMR) element. The conventional CPP-SV (TMR) element includes a multilayer magnetoresistive film 100 and an upper electrode 131 (also serving as a shield) and a lower electrode 121 (also serving as a shield) for causing current to flow through the magnetoresistive film 100. A lower metal film (terminal) 122, an antiferromagnetic pinning layer 123, a ferromagnetic pinned (fixed) layer 124, a non-magnetic layer (tunnel barrier layer) 125, a ferromagnetic free layer 126, and a cap layer 127 are stacked in order from the lower electrode 121 side. Reference numeral 128 denotes a hard magnetic layer, and reference numeral 129 denotes an insulating layer.
The cap layer 127 is a film protecting the magnetoresistive film 100. Because of the discontinuity of the formation process, a base having the magnetoresistive film 100 formed therein is placed in air before formation of an upper metal film 130 and the upper electrode (shield) 131. At this point, the cap layer 127 is provided in advance as a protection film in order to prevent the upper surface of the magnetoresistive film 100 from being oxidized in the air.
Conventionally, in order to reduce the contact resistance between the magnetoresistive film 100 and the upper electrode (shield) 131 or the upper metal film 130, part of the surface of the cap layer 127 in which resistance has increased because of oxidation is removed physically by etching before formation of the upper metal film 130 and the upper electrode (shield) 131, or a material difficult to oxidize, such as noble metal, is employed for the cap layer 127.
However, in the former method (physical removal), there is a problem in that a thick oxidation layer is formed, and that the film thickness controllability is low because of process-dependent variations in the formed oxidation layer.
According to the latter method, the oxidation layer to be removed is relatively thin. However, there is a problem in that the film thickness controllability is low because of high physical etching rates, and that tolerance to processing (such as resist patterning and oxygen ashing) cannot be obtained.
Accordingly, it is required to stabilize shape formation of elements, that is, to increase the yield of products, by selecting an appropriate material for the protection layer and selecting an effective element shape formation process.
According to aforementioned Japanese Laid-Open Patent Application No. 2003-198005, the need for removal of an oxidation film on the surface of a cap layer is eliminated by forming at least the surface part of the cap layer of a metal nitride, which is difficult to oxidize.